In my case two Rel Carbon Specials slightly defocused midrange vocals. I tried everything - repositioning 1000X or more, and following all guidelines with respect to volume and crossover settings. Every time I thought I had nailed the setup, I realized the subs weren’t actually engaged at all! I really tried everything I was advised to try and did nothing but experiment with them every day for a month after break in. I was able to return them, luckily.
Of course, this is just my unique experience. I think most people love them, especially at this level. The deep palpable bass was excellent and well integrated. But they also didn’t produce the dynamic and energetic higher frequency bass I was seeking for my very large room with high ceilings, leaving my B&W 802’s a bit underpowered for my space. But the most annoying thing was that smearing effect on the image at center stage. Moved to Audiovector R6 Arrete’s and these gave me all I was looking for with their dual downward firing active drivers - and I have zero desire to add subs now. Bass is now deep, defined, punchy and dynamic with my Diablo 300. Really has the palpable “growl” back in bass solos that I was missing. I don’t have that same bass you “feel” with the Rel’s, but I really don’t miss it at all, and I don’t think I was every really going for that really low end anyways.
Just my experience of course, which as I mentioned I think is unusual!